Astronomers Say They Finally Found Half the Universe’s Matter. It was clearly absent from view.

The apparent absence of half of the ordinary matter in the universe—known as baryonic matter—has long puzzled astronomers. This matter, which includes stars, planets, and gas, was expected to account for about 5% of the universe’s total energy density. However, despite extensive research, only a small amount could be observed in galaxies, stars, and other observable structures. Even the most advanced telescopes couldn't see the rest because they appeared to be hiding. The story has been altered by a groundbreaking discovery at this point. Surprisingly, the missing matter was hidden in plain sight, intertwined with the vast cosmic web that connects galaxies, according to researchers.

The universe’s ordinary matter is a small but critical piece of the cosmic puzzle, dwarfed by dark matter (27%) and dark energy (68%). While dark matter and dark energy remain mysterious, baryonic matter is tangible, composed of protons, neutrons, and electrons. Early universe measurements, derived from the cosmic microwave background and the abundance of light elements like helium, predicted a precise amount of baryonic matter. However, when astronomers tallied the matter in galaxies, star clusters, and interstellar gas, they were only able to account for approximately half of the expected quantity. This discrepancy, known as the “missing baryon problem,” has puzzled scientists for decades. Where was the other half?

The universe's vast network of filaments, walls, and voids, known as the cosmic web, turns out to be the source of the answer. These filaments, made of tenuous gas, connect galaxies like threads in a cosmic tapestry. The gas within them is so diffuse, with densities thousands of times lower than typical interstellar gas, that it’s nearly invisible to traditional telescopes. For years, scientists suspected that the missing baryons resided in these filaments, but detecting them was a formidable challenge. The gas is hot, with temperatures ranging from 100,000 to 10 million degrees Kelvin, and emits faint signals that are difficult to distinguish from background noise.

This occluded issue has finally been revealed thanks to recent advancements in observational methods. Using advanced X-ray telescopes and a technique called the Sunyaev-Zel’dovich effect, astronomers have mapped the elusive gas in the cosmic web. When hot electrons in the filamentary gas are scattered by photons from the cosmic microwave background, the Sunyaev-Zel'dovich effect occurs, resulting in minute distortions of the microwave background. By stacking data from thousands of galaxy pairs, researchers amplified these faint signals, revealing the presence of baryonic matter in the filaments. Both the University of Waterloo's and the Institut d'Astrophysique Spatiale's independent studies proved that the gas in these filaments is responsible for the absence of baryons.

This discovery is a triumph of persistence and technological innovation. Earlier attempts to detect the missing baryons focused on cooler gas clouds or relied on less sensitive instruments, yielding inconclusive results. The new observations, however, leveraged the unprecedented sensitivity of telescopes like the XMM-Newton and Planck satellites. By analyzing the distribution of gas between galaxies, astronomers not only located the missing matter but also confirmed that it matches the predicted baryon density from the early universe. This alignment strengthens our understanding of cosmic evolution, from the Big Bang to the formation of galaxies.

The implications of this finding are profound. The cosmic web’s filaments are not just repositories of missing matter; they are dynamic structures that fuel galaxy formation. Gas within these filaments flows toward galaxies, providing the raw material for new stars. Understanding the properties of this gas—its temperature, density, and composition—offers insights into how galaxies grow and evolve. In addition, the finding confirms the standard cosmological model, which holds that the structure of the universe was formed by billions of years of amplified density fluctuations in the early universe.

The missing baryon problem appears to be solved, but there are still questions. The cosmic web is vast, and mapping its full extent will require even more precise observations. Future missions, such as the Athena X-ray observatory, promise to reveal finer details about the gas’s behavior and its role in cosmic evolution. For the time being, astronomers are celebrating a significant milestone: the universe's missing matter, which was previously a perplexing mystery, was always there, waiting to be observed in the faint glow of the cosmic web.